Interpretive Summary: Septoria tritici blotch (STB), caused by the fungus Mycosphaerella graminicola (asexual stage: Septoria tritici), is one of the most destructive foliar diseases of wheat worldwide, particularly in Europe, north Africa, and North and South America. The disease spreads rapidly during cool, wet weather and is one of the top two or three diseases of wheat in many parts of the United States. Gene Stb2, derived from wheat cultivar Veranopolis, provides effective resistance against STB and previously was mapped to the short arm of wheat chromosome 3B. The purpose of this research was to find additional markers linked to the resistance gene to make it easier to use in wheat breeding programs. However, attempts to refine the map location of Stb2 could not confirm its reported location. Instead, based on characterization of progeny from a cross between resistant cultivar Veranopolis and a susceptible parent, Stb2 appears to be located on the short arm of wheat chromosome 1B, flanked by several molecular markers. These molecular markers can be used in wheat breeding programs to facilitate combining Stb2 with other resistance genes. This information will be useful to wheat geneticists to better understand the relationships among markers on wheat chromosome 1B. Plant pathologists and wheat breeders can use this information to transfer the gene into improved cultivars through marker-assisted selection at an early seedling stage rather than the tedious process of adult-plant inoculations. Correcting a previous mistake in the map location of Stb2 aids wheat geneticists in understanding the inheritance of resistance to STB, and will help identify other resistance genes that can be combined to develop resistance against multiple pests and pathogens in the future.

Technical Abstract:
Septoria tritici blotch (STB) is one of the most destructive foliar diseases in many of the wheat (Triticum aestivum L.) growing regions of the world. Gene Stb2, derived from cv. ‘Veranopolis’, provides effective resistance against STB. Attempts to refine the map location of this resistance gene could not confirm a previous report that Stb2 is on wheat chromosome 3BS. Instead, based on characterization of a doubled-haploid population derived from a cross between Veranopolis and susceptible line RAC875-2 and linkage analysis of the resistance phenotype to previously mapped SSR markers, we report that Stb2 is located on the short arm of wheat chromosome 1B, flanked by microsatellite loci Xwmc406 (with Xwmc500 tightly linked) and Xbarc008 (with Xwmc230 tightly linked) at map distances of 6 and 4.9 cM, respectively. These four co-dominant markers can be used in wheat breeding programs to facilitate combining Stb2 with other resistance genes. Other STB resistance genes, including Stb11, have been reported on chromosome arm1BS, with locus Xbarc008 as a diagnostic marker. Whether Stb2, Stb11 and the previously identified Stb11-like genes are the same, allelic, or different but closely linked genes has not been determined.